Let’s simplify this! Reverberation time is used to measure how long a sound stays present within a space after it occurs, and this determines whether there is too much noise or echo for occupant comfort and utility.
Physical space and the intended use are key factors in selecting an acceptable reverberation time for an area. As an example, in areas where we talk, we generally seek a short reverberation time so that we don’t hear multiple sounds simultaneously, which would lead to garbled speech. Conversely, in musical performance areas we prefer a long reverberation time so that the musical notes blend in together, rather than producing a dry, flat tone.
The human ear is most sensitive in the mid-frequency range, generally between 500 to 1,000Hz (the band where speech is produced). We control the reverberation time in a space by controlling the ratio of sound-absorptive to sound-reflective surfaces.
We can measure and predict reverberation time, and it is a great tool for predicting the performance of spaces prior to construction. Here some examples of recommended reverberation times based on the volume of the space and its intended use:
- Meeting room (850m3), recommended reverberation time 0.8 seconds
- Classroom (1,130m3), recommended reverberation time 0.8 seconds
- Theatre (2,830m3), recommended reverberation time 1.0 second
- Multi-use auditorium (5,665m3), recommended reverberation time 1.7 seconds
- Church [with organ music] (8,500m3), recommended reverberation 2.0 seconds
Music spaces need optimum acoustic performance, and this requires designers to consider reverberation, absorption, reflections, diffusion and volume. To achieve moderate reverberation time, there must be good sound reflections and diffusion close to the source. This has the effect of randomly scattering the sound waves and giving the musician the positive sensation that sound is coming from every direction.
The change in reverberation time is used to calculate the reverberant noise reduction. The table below shows an approximation of human sensitivity to the changes in reverberation time and the corresponding reverberant sound level. A 75% reduction in reverberation time will result in a 6dB change and this is the minimum required for the sound level change to be clearly discernible.
The Noise Reduction Coefficient (NRC) rating is an average of the absorption coefficient at 250, 500, 1000, and 2000Hz. In layman terms, the NRC shows the average percentage of sound that is absorbed by a material. An NRC of 0.85 generally states that the material absorbs, on average, 85% of the sound in the mid and high-frequency range.
Calculating the reverberation time in a space means you must know the area and NRC of each surface type. Tables of NRC ratings are available for a large variety of materials. Here are some examples:
- Gypsum board, NRC 0.05
- Empty wooden church pew (0.1 m3/seat), NRC 0.15
- Carpeted floor, NRC 0.35
- Occupied upholstered chair (0.14m3/seat), NRC 0.80
- Acoustic ceiling tiles, NRC 0.50 to 0.90
- Fiberglass wall panels, NRC 0.90
The benefit of analysing the expectations for the space up front, is that it can prevent too much being spent on treating it. Whilst you can’t really go wrong by adding as much absorption as possible, it is also important to consider the diminishing returns in doing so.
Winston Churchill once said: “We shape our buildings; thereafter they shape us.” The buildings where we work, live and play have an indelible impact on us and we need to develop and operate them this in mind.